Semiconductor nanoparticles, or quantum dots, are nanometer or micrometer-sized semiconductor structures in which one to a few thousand charge carriers, e.g., electrons, are confined, giving them a unique ability to emit visible or near infrared (IR) photons within a very narrow spectrum and with high efficiency. Because of this, semiconductor nanoparticles can be useful in numerous biological, genomic and proteomic applications, for example, as markers, as components of microchip arrays (biochips), and as conjugates for fluoroimmunoassays for in vitro and in vivo molecular imaging studies.
At present, fluorophores synthesized from organic molecules typically are used in such applications. While organic fluorophores have had some success, they tend to be unstable and gradually degrade when exposed to blue or UV excitation light, in a phenomenon known as photobleaching, or simply degrade with time. Further, such molecules typically emit light in the normal visible region, similar to that of other materials used in biological applications, resulting in a poor signal to noise ratio. Organic fluorophores that emit light in the near-infrared region are not generally commercially available and, hence, difficult to obtain.
Inorganic semiconductor nanoparticle materials synthesized from CdSe and ZnS-based materials are also used in biological, genomic and proteomic applications. Because such inorganic materials can be toxic when used in vivo, a coating of a protective material usually is necessary to render them useful in biological applications. However, as a result of the coating, which absorbs and/or reflects light, the fluorescent properties of the material are diminished.
Moreover, many inorganic nanostructures are hydrophobic by nature and consequently are not easily dissolved or suspended in aqueous solutions, rendering their use difficult in various applications, both in vitro and in vivo. Accordingly, there remains a need for better semiconductor nanoparticles, especially for use in fluorescence-based biological applications.